Flow structure and resistance over subaquaeous high‐ and low‐angle dunes

Abstract

A prominent control on the flow over subaqueous dunes is the slope of the downstream leeside. While previous work has focused on steep (~30°), asymmetric dunes with permanent flow separation, little is known about dunes with lower lee slope angles for which flow separation is absent or intermittent. Here we present a laboratory investigation where we systematically varied the dune lee slope, holding other geometric parameters and flow hydraulics constant, to explore effects on the turbulent flow field and flow resistance. Three sets of fixed dunes (lee slopes of 10°, 20°, and 30°) were separately installed in a 15 m long and 1 m wide flume and subjected to 0.20 m deep flow. Measurements consisted of high‐frequency, vertical profiles collected with a Laser Doppler Velocimeter. We show that the temporal and spatial occurrence of flow separation decreases with dune lee slope. Velocity gradients in the dune leeside depict a free shear layer downstream of the 30° dunes and a weaker shear layer closer to the bed for the 20° and 10° dunes. The decrease in velocity gradients leads to lower magnitude of turbulence production for gentle lee slopes. Aperiodic, strong ejection events dominate the shear layer but decrease in strength and frequency for low‐angle dunes. Flow resistance of dunes decreases with lee slope; the transition being nonlinear. Over the 10°, 20°, and 30° dunes, shear stress is 8%, 33%, and 90% greater than a flat bed, respectively. Our results demonstrate that dune lee slope plays an important but often ignored role in flow resistance.

The Importance of Splat Events to the Spatiotemporal Structure of Near‐Bed Fluid Velocity and Bed Load Motion Over Bed Forms: Laboratory Experiments Downstream of a Backward Facing Step

“…Additionally, flow separation, reattachment, and reacceleration change the temporal distribution of near‐bed fluid velocities at any given distance along the bed form profile (Figure c). Instead of being a narrow distribution that is characterized fairly well by the mean, flow separation and acceleration over bed forms increase the dispersion of the fluid velocity distribution and shear stress distributions (Figure c; Emadzadeh & Cheng, ; Jovic & Driver, ; Kwoll et al, ; Le et al, ; Nelson et al, ). Due to greater dispersion, it is possible to have both a mean shear stress that is below the critical shear stress as well as events that exceed that same critical shear stress (Figure c).…”

Three‐Dimensional Flow Above River Bedforms: Insights From Numerical Modeling of a Natural Dune Field (Río Paraná, Argentina)

“…The presence of a FSZ is also strongly related to the maximum angle of the slip face slope (Figure i). Lefebvre et al () and Kwoll et al () showed that over regular bedforms, the size of the flow separation increases with increasing slip face angle. In the present case, it is difficult to differentiate between the absence of flow separation and a shortening of the FSZ with decreasing maximum slip face angle.…”

“…Based on numerical modeling of flow over triangular bedforms, Lefebvre and Winter () suggested that flow separation is permanent for slip face angles steeper than 11–18°, depending on bedform relative height (bedform height compared to water depth). From laboratory measurements of flow over fixed bedforms, Kwoll et al () concluded that a permanent flow separation exists only over slip face angles of 30°, and a small intermittent FSZ is present over bedforms with slip faces of 10° and 20°. These recent results based on systematic studies confirmed previous results indicating that flow separation is permanent for lee side slopes larger than 10–20° (Best & Kostaschuk, ; Kostaschuk & Villard, ; Paarlberg et al, ).…”

Observations of Coherent Flow Structures Over Subaqueous High‐ and Low‐ Angle Dunes

“…Lee side slopes of these low‐angle dunes are typically ≪30°, often ~10° (e.g., Kostaschuk and Villard, , ; Hendershot et al, ; Roden, ; Smith & McLean, ), and do not exhibit permanent flow separation. Instead, the flow recirculation region over low‐angle dunes is replaced by a zone of decelerated flow with intermittent separation (Best & Kostaschuk, ; Carling et al, ; Kwoll et al, ; Lefebvre & Winter, ; Roden, ; Smith & McLean, ). The generation mechanism for large‐scale CFS in the absence of permanent flow separation has not been identified, despite observations from deep rivers and estuaries that show significant transport of suspended sediment associated with CFS over low‐angle dunes (Hendershot et al, ; Kostaschuk, ; Kostaschuk & Villard, ; Kwoll et al, ; Smith & McLean, ).…”

“…Some recent advances have been made in laboratory investigations of flow over low‐angle dunes and negative steps with low step angle (Best, Kostaschuk, & Villard, ; Best & Kostaschuk, ; Kwoll et al, ; Motamedi et al, ; Motamedi, Afzalimehr, Harb, et al, ; Motamedi, Afzalimehr, Singh, et al, ; Ruck and Makiola, ; Sukhodolov, Fedele, & Rhoads, ). Ruck and Makiola () and Sukhodolov et al () documented wake formation and hence turbulence production downstream of low‐angle negative steps (10° and 15°) and dune lee slopes of 10°, respectively, in the absence of permanent flow separation.…”